What is a stock index?

A stock index is a basket of selected shares whose combined value tracks a slice of a market. It gives a single number that summarises how that market or sector is performing, and underpins index funds and futures.

What is the difference between market-cap and price weighting?

In a market-cap weighted index like the S&P 500, larger companies move the index more. In a price-weighted index like the Dow Jones or Nikkei 225, higher-priced shares carry more weight regardless of company size.

Why do indices have round component counts?

Many indices fix the number of constituents by rule — the FTSE 100 holds the largest 100 UK companies, the DAX holds 40. The S&P 500 targets about 500 but the exact count drifts slightly with dual-listed shares.

Can I invest in an index directly?

Not in the index itself, but you can buy index funds or ETFs that replicate it, or trade index futures and options. These track the index's value closely minus fees.

How current is the component count?

Counts reflect each index's published methodology at publication. Index providers rebalance periodically and occasionally change the target size, so verify with the provider for an exact figure.

Major Stock Index Reference

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Benchmark equity indices at a glance

This reference lists the world’s most-watched stock market indices — the S&P 500, FTSE 100, DAX, Nikkei 225 and more — together with the exchange they trade on, the country they represent, the approximate number of constituent companies, and the weighting method that drives how they move.

How it works

A stock index aggregates the prices of its constituent shares into a single number. The weighting method determines each company’s influence. In a market-capitalisation weighted index, a company’s weight equals its market value relative to the whole basket, so giants dominate. In a price-weighted index, the weight depends only on share price, so a high-priced stock sways the index even if the company is modest in size:

cap-weighted: weight_i = (price_i × shares_i) / Σ(price × shares)
price-weighted: weight_i = price_i / Σ(price)

Providers rebalance the basket on a schedule, adding and dropping companies to keep the index representative.

Tips and notes

Sort by Components to compare the breadth of each index: broad indices like the S&P 500 capture a whole market, while narrow ones like the Dow’s 30 names track only blue chips.
Weighting matters when you compare moves: a price-weighted index can be swung by one expensive stock, which is why the Nikkei and Dow sometimes diverge from cap-weighted peers.
Use the exchange and country columns to map an index to the economy it represents before reading it as a sentiment gauge.